Harry Potter and the Deathly Hallows is a bit murky regarding the Slytherin students and their role in the final Battle of Hogwarts. We know the following:
- Professor McGonagall advises Professor Slughorn that she expects the Slytherins in the Great Hall following Snape's departure. She warns Slughorn that it is time for Slytherin House to decide upon its loyalties, and that if the Slytherins resist the attempt to fight Voldemort, they will duel to the death. (Deathly Hallows - Pages 599-600 US Hardcover)
- Slytherin Pansy Parkinson attempts to have Harry turned over to Voldemort when Voldemort threatens to attack Hogwarts. McGonagall sends all of Slytherin House from the Great Hall. (Deathly Hallows - Page 610 US Hardcover)
Voldemort advises Lucius Malfoy that all the Slytherins have joined Voldemort's side. (Deathly Hallows - Page 641 US Hardcover)
Phineas Nigellus, following the Battle of Hogwarts and Voldemort's death, says to Harry when Harry visits the Headmaster's Office, "And let it be known that Slytherin House played its part! Let our contribution not be forgotten!" (Deathly Hallows - Page 747 US Hardcover)
What role did the Slytherins ultimately play in the Battle of Hogwarts? Did they resist Voldemort, assist him, or play both sides of the fence?